This invention relates to lasers and nonlinear optics and, more particularly, uses several laser beams in conjunction with sum-frequency generation to produce a high-power beam having a desired wavelength, e.g., the sodium D2a spectral line.
The earth's atmosphere distorts light traveling through it because of time and spatially varying fluctuations in its refractive index. Such distortion limits the resolution of terrestrial telescopes, and was the motivation for launching the successful, but very expensive, Hubble telescope into earth orbit. However, ground-based telescopes can achieve a resolution surpassing that of the Hubble space telescope by means of adaptive optics, a technique whereby the surface of a deformable telescope mirror is changed as a function of time to compensate for atmospheric distortion.
Measuring the distortion requires that there be a bright optical source in the sky, such as a bright star, located close to the object to be observed. Since natural stars of sufficient brightness are too rare to permit compensated imaging except over a tiny fraction of the sky, an alternative is to produce one or more artificial guide stars by means of back-scattered light from a laser beam or similar source of coherent radiation directed from the site of the telescope into the area of the sky proximate to the object to be observed. Such guide stars can be produced by exciting resonance fluorescence at a 589-nm wavelength, i.e., at the sodium D2a spectral line, from a layer of sodium atoms that circumscribes the earth in the mesosphere at an altitude of approximately 90 km.
Although high-power dye lasers can be operated at this wavelength to excite the sodium layer, they are inefficient, bulky, expensive, and use chemicals which are toxic and which degrade over time, concomitantly resulting in degradation of performance. The desirability of an all-solid-state sodium beacon excitation source has long been recognized. W. Happer, G. J. MacDonald, C. E. Max, and F. J. Dyson, “Atmospheric-turbulence compensation by resonant optical backscattering from the sodium layer in the upper atmosphere,” J. Opt. Soc. Am. A, Vol. 11, No. 2, pp. 263–276 (January 1994). However, the goal of building a reliable and efficient device that generates sufficient power with good beam quality has been elusive, despite over a decade of endeavor.
By a natural coincidence, sodium resonance radiation at λd=589 nm can be produced by sum-frequency generation of the Nd:YAG laser lines at λp=1064 nm and λs=1319 nm. This has motivated the development of pulsed sources using single-pass sum-frequency generation in lithium triborate (LiB3O5). T. Jeys and V. Daneu, “Diode-pumped, Nd:YAG source of sodium-resonance radiation for atmospheric adaptive optics,” ESO Workshop on Laser Technology for Laser Guide Star Adaptive Optics Astronomy, Garching, Germany (Jun. 23–26, 1997); and a low-power continuous-wave source using doubly resonant sum-frequency generation in lithium niobate (LiNbO3). J. D. Vance, C.-Y. She, and H. Moosmuller, “Continuous-wave, all-solid-state, single frequency 400-mW source at 589 nm based on doubly resonant sum-frequency mixing in a monolithic lithium niobate resonator,” Applied Optics, Vol. 37, No. 2, pp. 4891–4896 (Jul. 20, 1998). The pulsed sources have proved too unreliable for use in conjunction with adaptive optics, whereas absorption in lithium niobate precludes the generation of high power in this crystal.
Various theoretical studies have been completed comparing the efficiency of different pulse formats, including continuous wave. J. M. Telle, P. W. Milonni, and P. D. Hillman, “Comparison of pump-laser characteristics for producing a mesospheric sodium guidestar for adaptive optical systems on large aperture telescopes,” in High-Power Lasers, S. Basu, Editor, Proceedings of SPIE, Vol. 3264, pp. 37–42 (January 1998).
The advantages of continuous wave are high duty cycle and narrow bandwidth. Both effects enhance the sodium fluorescence return, as long as saturation is avoided. Continuous wave sources have been used, but have only been able to generate low beam power, i.e., power too low for astronomical imaging using adaptive optics to correct atmospheric distortion.
It follows that there is a need in the art for a terrestrial apparatus capable of generating an artificial guide star by means of back-scattered light from a laser beam or similar source of coherent radiation and, more particularly, by creating such a guide star by exciting resonance fluorescence from a layer of sodium atoms that circumscribes the earth in the mesosphere. The present invention has fulfilled this need in the art.